Sialon-based ceramic materials are well known for their good mechanical and physicochemical properties, in particular their good resistance to a corrosive atmosphere (in particular in an oxidizing environment).
Various methods are known for making Sialon, mostly for obtaining solid parts.
Known methods generally make use either of direct synthesis from a mixture of powders, or else they make use of reduction of an aluminosilicate material.
For direct synthesis, a mixture of powdered silicon nitride, aluminum nitride, silica, and alumina (Si.sub.3 N.sub.4, AlN, SiO.sub.2, and Al.sub.2 O.sub.3) is compacted under high pressure and is sintered at very high temperature (1700.degree. C. to 1800.degree. C.). Sintering may be performed under load, or with an incorporated additive such as yttrium oxide (Y.sub.2 O.sub.3). That method suffers from the drawbacks of requiting raw materials that are quite expensive, and of requiting treatment at very high temperature. In addition, although it is suitable for obtaining parts that are dense, it is unsuitable for forming Sialon-based coatings on substrates.
Several methods of obtaining Sialon by reducing an aluminosilicate material make use of carbon as a reducing agent. The aluminosilicate material may be constituted by a mechanical mixture of oxides such as silica and alumina, or by a synthetic mixture, e.g. obtained by the sol-gel technique, or else by a natural mixture, in particular a compound selected from those known overall under the term "clays".
Reduction by means of carbon requires intimate contact between the aluminosilicate material and the carbon. This can be obtained by intimately mixing the precursor with carbon powder (carbon black) or chemically by using insertion compounds. In the latter case, the precursor used is a clay whose characteristic scaly structure makes it possible to insert organic molecules (e.g. acrylonitrile) between scales, which molecules, after being subjected to pyrolysis, constitute the source of carbon required for reduction. Which method makes it possible, by carbon reduction, to obtain a .beta.-Sialon at a temperature (about 1100.degree. C.) that is lower than the temperature required (about 1350.degree. C. to 1450.degree. C.) when starting from a mechanical mixture of aluminosilicate material and of carbon powder.
A drawback of the above carbon reduction methods lies in the difficulty of accurately controlling the carbon content, and thus the amount of reduction. Another drawback, that results from the inclusion of carbon in the starting compound, consists in the resulting ceramic being porous because of the inevitable evolution of carbon monoxide during reduction.
Other known methods propose obtaining a Sialon by reducing an aluminosilicate material by means of ammoniac (NH.sub.3) or a mixture of ammoniac and hydrogen H.sub.2. As before, the precursor may be a mechanical mixture of SiO.sub.2 and Al.sub.2.sub.O.sub.3 powders or it may be a clay. Such methods make it possible to work at a temperature of about 1000.degree. C., i.e. a temperature that is lower than those used in carbon reduction. Nevertheless, the Applicant has observed that such methods give rise, in fact, to a ceramic that is essentially Constituted not of Sialon, but of a mixture of mullite and of cristobalite.
Finally, other known methods propose using a polyaluminocarbosilane material to obtain Sialon by treatment under ammonia at about 1400.degree. C. to 1500.degree. C., or using mixtures of opalite and ammonium to obtain Sialon by electrical discharge under a nitrogen atmosphere at 1750.degree. C.